For the last two decades, resin materials known as ionomers have been used extensively as cover stock materials for golf balls. Over the years, it has been found that such ionomers can be used to form very durable covers for golf balls, while also providing acceptable in-play characteristics such as spin rate, feel and initial velocity.
These resins are well known and are commercially available under a variety of tradenames such as SURLYN.RTM. (DuPont) and IOTEK.RTM. (Exxon). Presently, there are more than 50 grades of ionomers commercially available having a wide range of properties which vary according to a variety of parameters such as the type and amount of metal cations, molecular weight and composition of the base resin (e.g. the relative content of ethylene and methacrylic and/or acrylic acid groups).
Generally speaking, ionomers are obtained by providing a cross metallic bond to polymers of monoolefin with at least one member selected from the group consisting of unsaturated mono- or di-carboxylic acids having 3 to 12 carbon atoms and esters thereof (the polymer contains 1 to 50% by weight of the unsaturated mono- or di-carboxylic acid and/or ester thereof). More particularly, such acid-containing ethylene copolymer ionomer component includes E/X/Y copolymers where E is ethylene, X is a softening comonomer such as acrylate or methacrylate present in 0-50 (preferably 0-25, most preferably 0-20), weight percent of the polymer, and Y is acrylic or methacrylic acid present in 5-35 (preferably 10-35, more preferably at least about 16-35, most preferably at least about 16-20) weight percent of the polymer, wherein the acid moiety is neutralized 1-90% (preferably at least 40%, most preferably at least about 60%) to form an ionomer by a cation such as lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc or aluminum, or a combination of such cations. Specific acid-containing ethylene copolymers include ethylene/acrylic acid, ethylene/methacrylic acid, ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylic acid/n-butyl acrylate, ethylene/methacrylic acid/iso-butyl acrylate, ethylene/acrylic acid/iso-butyl acrylate, ethylene/methacrylic acid/n-butyl methacrylate, ethylene/acrylic acid/methyl methacrylate, ethylene/acrylic acid/methyl acrylate, ethylene/methacrylic acid/methyl acrylate, ethylene/methacrylic acid/methyl methacrylate, and ethylene/acrylic acid/n-butyl methacrylate. Preferred acid-containing ethylene copolymers include ethylene/methacrylic acid, ethylene/acrylic acid, ethylene/methacrylic acid/n-butyl acrylate, ethylene/acrylic acid/n-butyl acrylate, ethylene/methacrylic acid/methyl acrylate and ethylene/acrylic acid/methyl acrylate copolymers. The most preferred acid-containing ethylene copolymers are ethylene/methacrylic acid, ethylene/acrylic acid, ethylene/(meth)acrylic acid/n-butyl acrylate, ethylene/(meth)acrylic acid/ethyl acrylate, and ethylene/(meth)acrylic acid/methyl acrylate copolymers.
The manner in which the ionomers are made is well known in the art and is described in a variety of references, including, e.g., U.S. Pat. No. 3,262,272.
Ionomers are generally colorless compounds and therefore it is necessary to add materials such as coloring agents in order to impart a suitable color to a golf ball cover composition formed from an ionomer blend. Suitable coloring agents include pigments such as titanium dioxide (TiO.sub.2), Zinc Oxide (ZnO) and Calcium Oxide (CaO), with TiO.sub.2 generally being a preferred material.
In order to achieve uniform color in the cover composition, it is important that the coloring agent be thoroughly mixed and well dispersed throughout the base ionomer resin. However, such dispersion is not easily achieved, as coloring agents, especially those having a relatively small particle size such as TiO.sub.2, do not readily disperse throughout large batches of the base ionomer resin.
One conventional method of obtaining an acceptable dispersion of coloring agent throughout the base ionomer resin is to form a color concentrate containing a coloring agent and admixing the color concentrate with the base ionomer resin blend. The color concentrate is formed by mixing the coloring agent with a carrier ionomer resin. Any conventional mixing method may be employed to form the color concentrate. A preferred method is mixing the coloring agent and the carrier ionomer resin in a twin screw extruder and pelletizing the resulting extrudate, thereby forming pellets of the color concentrate composition.
The color concentrate pellets are subsequently admixed with pellets of the base ionomer resin, typically in an injection molding machine to form a cover blend for forming a golf ball cover. The cover blend comprises about 99 to about 90% by weight base ionomer resin and about 1 to about 10% by weight color concentrate; preferably about 98 to about 93% by weight base ionomer resin and about 2 to about 7% by weight color concentrate and most preferably about 96 to about 95% by weight base ionomer resin and about 4 to about 5% by weight color concentrate. Unless otherwise noted, "% by weight" as used herein refers to the percentage by total weight of the cover layer blend.
Additional components conventionally added to cover compositions include ultraviolet light stabilizers and/or absorbers, optical brighteners, fluorescent pigments, dyes, processing aids and fillers. The total amount of such additional components is typically about 1 to about 10% by weight, preferably about 2 to about 8% by weight, and more preferably about 5% by weight.
However, golf balls having covers formed from a blend of base ionomer resin and a color concentrate formed from a carrier ionomer resin and a coloring agent have, on occasion, been observed to exhibit delamination of the cover layer and/or an increased occurrence in cover failure along the seam of the cover. Delamination and seam failure are generally recognized in the golf ball art to be indicative of incomplete mixing and/or incompatibility of the components of the cover compositions.
Therefore, there exists a need for a method of making golf ball cover blends, as well as covers made from such blends, which avoid or prevent the occurrence of cover layer delamination and/or the occurrence of seam failures, thereby increasing the durability of the golf ball cover and extending the useful life of the golf ball.